German patent publication 2,945,472 discloses a ventilating apparatus having a closed respiratory loop wherein respiratory gas is continuously metered into the breathing loop via a metering unit. The ventilating apparatus includes a ventilating bellows which compresses during the inspiration phase and decompresses during the expiration phase. The ventilation of a patient connected to the respiratory loop is made possible by means of this ventilating bellows. The stroke volume to be supplied to the patient can be adjusted by an adjusting device which limits the stroke of the bellows.
It is disadvantageous with this known ventilating apparatus that the stroke volume adjusted with the adjusting device is increased by the respiratory gas which flows in continuously from the metering device into the breathing loop. This influence becomes that much greater the lower the setting of the stroke volume is and the more respiratory gas is supplied by the metering unit to the breathing loop.
U.S. Pat. No. 5,119,810 is incorporated herein by reference and discloses a ventilating apparatus wherein the supply of fresh respiratory gas is interrupted during the inspiration phase by means of a valve in the connecting line between the metering unit and the breathing loop. It is disadvantageous that the metering unit must be designed either for a high gas flow, which occurs during the exhalation phase, or a buffer volume has to be provided rearward of the metering unit from which the required respiratory gas can be taken during the exhalation phase.